Since the divestiture of the American Telephone & Telegraph Company in 1984, the Regional Bell Holding Companies (RBHCs) have focused their efforts on cutting operating costs, upgrading their networks, providing new high speed services, and interconnecting their networks to long-distance companies and international carriers. One of the tools the RBHCs have chosen to help them achieve these goals is the Synchronous Optical Network (SONET). SONET is both a standard and a set of specifications for building high speed, digital communications networks that run over fiberoptic cables while interfacing with existing electrical protocols and asynchronous transmission equipment. Fiberoptics has revolutionized telecommunications in view of the large bandwidth availability (currently estimated in the hundreds of gigabits per second) which continues to increase with technological advances such as wave-division multiplexing and similar developments in light polarization and dispersion-shifted fibers.
As those skilled in the art will recognize, SONET specifies a digital hierarchy based on Optical Carrier (OC) rather than electrical levels. SONET does define Synchronous Transport Signals (STS), however, which are electrical interfaces used as the multiplexing mechanisms within SONET Network Elements (NEs). Network elements combine STS-1s as needed up to STS-N where N is the number of STS-1s, then convert the total electrical multiplex to an Optical Carrier and transmit it over optical fiber. SONET is multiplexed at the byte level, allowing services to be dynamically placed into the broadband STS for transport. The basic SONET of 64 Kbps per byte is the same speed as the conceptual voice channel DS0 allowing SONET to easily integrate all currently used digital services into the optical hierarchy.
One of the principal benefits of SONET is that it allows for the direct multiplexing of current network services, such as DS1, DS1C, DS2, and DS3 into the synchronous payload of STS-1. As those skilled in the art will recognize, the above rates, as in the case of most defined rates, were developed based on existing transmission systems. For example, the DS1 and DS2 signal rates (1.544 million bits per second and 6.312 million bits per second) are the transmission rates of the T1 and T2 wire pair carrier systems. Initially, one multiplexer, called an M12, was used to combined four DS1 channels into a DS2, and a second multiplexer, called an M23, was used to combine seven DS2 channels into a DS3. Presently, most networks use a single multiplexer termed an M13, which combines twenty-eight DS1 channels into a DS3. Of course, one of the key attributes of these previous multiplexer designs is that they permit DS1 signals to be timed independently, i.e. asynchronous multiplexing. Bits can therefore be sent at different transmission rates because individual channels need not be synchronized to a common timing source.
The asynchronous DS3 multiplexing standard was implemented in the days when most networks utilized analog technology and the few digital systems in existence generated their own clocking systems. Significantly, the transmission specifications for DS1 signals specify that the bit rate is 1.544 million bits per second, plus or minus 75 bps. To compensate for this range, additional bits must therefore be "stuffed" into each DS1 signal before they are multiplexed to a higher rate. Again, as those skilled in the art will recognize, while bit stuffing supports independently clocked input signals, it also makes it nearly impossible to locate individual DS1 or DS0 channels within a DS3 bit stream. To extract a single channel, a DS3 signal would need to first be demultiplexed through M13 components into twenty-eight DS1s before the channels could be switched or rearranged. As a result, the process of adding or deleting channels is expensive.
In contrast to asynchronous multiplexing, the SONET standard defines a viable alternative which supports greater capacity and efficiency. In the SONET multiplexing format, the basic signal transmission rate --STS-1--operates at 51.84 million bits per second. AN STS-1 can carry 28 DS1 signals or one asynchronous DS3. STS-1 signals are then multiplexed to produce higher bit rates--STS-2, STS-3, etc. As referenced above, the other term used to define the SONET signal levels is optical carrier. The bit rates are the same in each case, so the bit rate of the STS-1 equals the bit rate of the OC-1. The only difference is the type of signal that is being referenced. For example, if the signal is in an electrical format, it is referred to as an STS. Similarly, if the signal is in an optical format--compatible with a fiber medium--it is referred to as an OC.
The SONET standards define an alternative to asynchronous DS3 multiplexing, which describes how to divided STS signals into lower speed increments, i.e. virtual tributaries. The major advantage of synchronous multiplexing is that when DS1 and other low-speed channels are multiplexed directly into the STS format, the lower speed channels can be identified and reconfigured for drop-and-insert. As a result, the drop-and-insert process can be done easier with less expensive hardware then the back-to-back M13 multiplexers used in asynchronous multiplexing.
Because of the large bandwidth availability in fiber, and the growing volume of data traffic, disruptions from link and node failures due to cable cuts, for example, become increasingly serious. Network survivability has therefore become a major concern for SONET designers and has fueled interest in what is known in the art as "ring" architectures. Such architectures take advantage of the capability provided by synchronous multiplexing in SONET to eliminate the need to backhaul traffic to central hubs. Thus, at each switching office, the SONET transport node directly accesses the required time slots in the bit stream through the use of modified Add-Drop Multiplexers (ADM). The SONET ring topology permits the creation of highly survivable networks which are viewed in the communications industry as essential for obtaining business for critical data communications.
In most cases, the deployment of SONET rings results in cost savings since it is far less expensive for carriers to install a fiber ring then to deploy point-to-point links. Consider, for example, a rural route, where linking remote terminals to a central office in a point-to-point application would require six multiplexers--one at each site and at the Central Office (CO) for each route--and six fibers, two to each site. In a ring topology, all that is required is one multiplexer at the CO and two fibers that go through a multiplexer at each site for a total of four multiplexers and two fibers. Significantly, in the ring topology, working or service traffic is routed in one direction only. If that fiber fails, traffic is rerouted on a protection fiber to flow in the opposite direction. In this manner, working traffic bypasses the failure to get to its proper destination.
Against this background, it can be readily appreciated that there is significant debate in the communications industry regarding the type and location of rings, and in particular, Self-Healing Rings (SHR) to deploy. There is similarly significant debate as to how to best assign traffic to the SONET rings once they have been deployed.
On this latter point, those skilled in the art have long recognized that current equipment available for traffic routing on SONET rings and, in particular, DS1 traffic, does not make full use of the available ring capacity. The problem of packing DS1 traffic into STS-1 time slots is combinatorially very difficult. Indeed, it becomes even more difficult if one tries to choose the packing in a way which minimizes the cost of multiplexing equipment, i.e. Terminal multiplexers, Add/Drop multiplexers and their corresponding interface ports. See, for example, the technical note "Engineering SONET Rings II: The Sequel", AN-93-003, April 1993, wherein the Fujitsu authors state that there is "no absolute solution" to the problem and provide ad hoc ideas for solving it.
Consequently, a need has developed for an optimizing tool which will provide network planners the ability to decide whether a given set of traffic and, in particular, DS1 demand may be packed on a single SONET ring. Still further, a need has developed to provide such a tool wherein the user may determine the minimum cost packing if there is more than a few DS1 demands.
Still further, a need has developed for such a system wherein the user may determine the optimized solution for traffic routing and, in particular, the number of multiplexers and interface ports.